In many crystal lasers used in coherent light amplifiers and oscillators a light-emitting element is added as a dopant to a material that serves as a crystalline host. The characteristics of a crystal laser depend on the dopant and crystalline host selected. Light emitting dopant materials include all the trivalent rare earth ions.
All laser materials have characteristic energy levels and transitions so that photons are emitted at certain transitions when electrons drop from excited states to lower states. Likewise, the materials absorb light at characteristic wavelengths when they are in the ground state or other low levels.
Light absorption can be at a narrow or broad range of wavelengths depending on the transitions involved. Laser operation at the holmium 5I6 to 5I7, and 5I7 to 5I8 transitions have been reported at near 2.9 microns (μm) and 2.1 microns (μm) in several different host crystals. Consequently, laser performances using broad spectral emission pump sources have been poor except where additional sensitizer (co-dopant) ions have been used. However, the use of sensitizer ions has its shortcomings.
Thulium (Tm) sensitized holmium doped laser materials have proven to be quite useful but have several disadvantages. For example, the near energy resonance between the Tm 3F4 and Ho 5I7 states results in incomplete energy transfer from the sensitizer ions. At room temperature with otherwise optimal densities of sensitizer, transfer of only approximately 60% of the stored excitation density to holmium ions can occur. This incomplete transfer proportionally increases the already high lasing thresholds associated with holmium and increases thermal loading. Further, the interaction of Tm 3F4 and Ho 5I7 meta-stable ions create a detrimental up conversion loss process that severely limits energy storage lifetimes and small signal gains. In addition, the short pulse performance of Tm/Ho lasers are limited by the up conversion loss process and the relatively slow energy transfer from Tm 3F4 and Ho 5I7. Finally, the thermal loading of the Tm/Ho laser material is increased by the incomplete energy transfer and up conversion losses, thereby limiting the utility of such material for average power production. Other sensitizer co-dopants also create problems.
Thus, there is a need in the prior art for a way to improve lasing thresholds associated with holmium doped crystal lasers of oscillators and amplifiers, while reducing thermal loading, reducing up conversion losses and minimizing other problems.